Recently, along with the explosive increase of information and data handled in companies and government offices, the amount of data processed in large-scale computers such as host computers or storage subsystems coupled to servers and host computers or the capacity of data stored therein are increasing rapidly. Unlike information processing systems such as servers, storage subsystems are systems specialized for storing data, and a large number of hard disks which are data storage media having high reliability and large capacity are used, which are managed and operated via a RAID (Redundant Array of Independent (Inexpensive) Disks) method.
Further, the reduction of costs of storage subsystems is advancing recently, and along therewith, the hard disk drives (hereinafter referred to as HDDs) used in the subsystem are exchanged from the expensive and highly reliable fiber channel (hereinafter referred to as FC) type to inexpensive SAS (Serial Attached SCSI) type, and further, a SATA (Serial ATA) type drive which is even more inexpensive than the SAS are becoming more and more popular.
However, since the SAS disks and SATA disks are inexpensive, they have lower reliability than FC disks. Therefore, the simple replacement of HDDs in the storage subsystem from the FC disks to SAS disks or SATA disks will deteriorate the reliability of the whole storage subsystem.
Moreover, the recent advancement of the high recording density technique has led to the increased capacity of HDDs. Therefore, there is fear that the frequencies of occurrence of non-recoverable or recoverable errors caused by physical defects such as flaws (defects formed during fabrication or cuts caused by head contact) on the media which is the recording section of the HDD or data read failures caused by insufficient recording are increased. Therefore, in order to prevent the loss of data caused by defects and the like, as shown in patent literature 1, verification processing (CRC (Cyclic Redundancy Check) during verification operation and the like) on the whole storage area within the HDD to detect defective portions and defective areas or data restoration such as replacement processing for replacing the defective areas are executed during operation of the storage subsystem.
The storage subsystem executes verification concurrently while processing data access requests from the host computers. Such verification is called online verification, which is executed asynchronously as the aforementioned data access processing. As a prior art technique regarding online verification, patent literature 1 discloses performing online verification processing via different cycles on the system area and the data area.